The regulator for U.S. housing finance giants Fannie Mae and Freddie Mac told the two firms on Wednesday to provide more support to low-income Americans taking out mortgages and refinancing home loans.

The Federal Housing Finance Agency released goals for the two government-controlled firms for 2015-2017 that would advance agency chief Mel Watt's aim to widen access to housing credit.

The rules direct Fannie Mae and Freddie Mac to expand the number of loans they back for low-income families to 24 percent of the their purchases of single-family home mortgages over the period, up from a target of 23 percent in 2014.

FHFA also asked each firm to make mortgages refinanced by low-income families a bigger share of their refinancing purchases, and to increase the number of mortgages they buy for multi-family properties each year.

Fannie Mae and Freddie Mac have been controlled by the U.S. government since taxpayers bailed them out in 2008 during the housing market implosion.

The two firms don't lend money directly, but buy mortgages from lenders and sell them as packaged securities with a government guarantee. They back most new U.S. mortgages, and their purchases are a major driver of credit access.

Boosting support for low-income borrowers, however, could stir controversy in the U.S. Congress. Many Republican lawmakers think Fannie Mae and Freddie Mac contributed to the housing bubble and 2007-2009 financial crisis with policies aimed at supporting mortgage access for the poor.